Who is Participating?

You can't add to the official shapes, but there are a couple of work arounds.

Draw up the shapes in a drawing tool and drop them onto a page called "Shapes". You will then need to copy and paste the images to where you need them.

A second workaround is to rough draw your shapes in OneNote with a pen and create the final (shaded) drawing in a drawing tool. (I actually do the drawings in PPT because it is easier for me - many of the shapes you need are in the drawing tools there and shading is easier.)

One other note: With OneNote for Win10 becoming the only supported OneNote soon, you will want to get used to these work arounds. The drawing tools in that version are not even as good as the ones in OneNote 2016. #MyTwoCents